This invention relates generally to optical telecommunications networks, and in particular to methods and apparatus for installing a loose bundle of guide tubes, through which fiber optic communications cables are to be routed, within a protective conduit such as an underground duct.
Various factors must be considered when fiber optic cable is installed in a conduit. A major concern is avoidance of damage to the cable during installation. Damage can occur in a variety of ways, namely 1) radial crushing the cable with installation equipment; 2) severe bending, twisting, flexing or stretching damage due to excessive forces applied during installation; 3) damaging the protective cable jacket, such as by abrasion, cracking or cutting the outer protective layer; and 4) long term exposure of the cable to environmental factors which cause thermal cycling.
Another concern is ease of installation and the desire for a reduction in the amount of time needed to install the cable. Also, there is a concern to avoid splices in the cable as much as possible. Splices are time consuming to make and may lead to a decrease in cable performance. Generally, it is desirable to install the longest continuous length of cable possible to reduce the number of splices needed for the desired cable run.
Protective cable ducts have been channelized in an effort to satisfy these concerns. For this purpose a tube, whose interior may have a lower coefficient of friction than the existing duct, is installed in the existing protective duct, thereby establishing a separate channel in which cable, optionally at a later time, can be blown or pulled through the protective duct over a greater length. It may also be desirable to install in an existing duct a larger number of tubes with a smaller cross section than that of the existing duct if it is desired to use each of the smaller tubes as a separate channel or subduct for single- or multi-core copper or glass fiber cables. Further, it may be necessary to install in an existing duct a protective tube with a water barrier, so that in the existing duct, whose interior gradually fills up with water through diffusion, a waterproof conduit is created by means of the second tube, this waterproof conduit allowing the routing of cables without a water shield.
An early approach to duct channelization is described in the article xe2x80x9cSubducts: The answer to Honolulu""s growing painsxe2x80x9d by Herman S. L. Hu and Ronald T. Miyahira in Telephony, Apr. 7, 1980, pages 23-35. That arrangement accommodates fiber optic cables in a separate tube or subduct, with four guide channels being formed by partitioning a primary duct forming part of a primary duct network. As more telecommunications connections are needed, a fiber optic cable is pulled by a rope or winching wire through one of the channels, without the new fiber optic cable being hampered during installation by cables already present. At branches, a joint or splice is made in the fiber optic cable. EP-A-0,108,590 to Reeve describes a ducting network, the ducts of which have previously been provided with a number of separate channels, allowing a separate lightweight and flexible fiber optic member to be blown in by compressed air in each channel without armor or water barrier. The duct provided with channels protects the fiber optic members against external influences, such as moisture and the like. In this way, a network with individual fiber optic members to each customer is created, with the fiber optic members being arranged in parallel channels up to the branches.
U.S. Pat. Nos. 4,850,569 and 4,934,662 to Griffioen et al. describe combining high speed air flow with a pushing force applied at the entry end of the conduit to install a traditional (i.e. with non-negligible stiffness) cable. U.S. Pat. Nos. 5,197,715 and 5,474,277 to Griffioen further describe the use of a guide shuttle attached to the lead end of the cable which adds a tension force on the lead end of the cable, in addition to the motive forces applied to the cable via the high speed moving air.
These techniques also have been used advantageously for installing channelization guide tubes in an existing protective duct. For example, in U.S. Pat. No. 5,884,384 to Griffioen, channelization is achieved by installing a bundle of guide tubes or subducts in an existing protective duct by means a fluid under pressure, for instance compressed air, together with a pushing force exerted on the guide tubes as they enter the protective duct.
In the air blowing/pushing technique the air drag propelling forces on the bundle are distributed over the entire length of the guide tubes, and the longitudinal forces imposed on the guide tubes are kept low. For making the most productive use of available underground duct space, it has been the conventional practice during the initial installation to fill the protective duct as completely as possible with channelization guide tubes of various diameters to accommodate present and anticipated cable branching/drop requirements. In previous guide tube installations, the size and number of guide tubes have been selected to provide close to 100 percent filling of the protective duct. However, it was found in practice that such jobs incur increased installation time, along with a reduction of the overall bundle length that can be blown in continuously, thus requiring more guide tube joints, more duct junctions and, last but not least, a shorter maximal distance between handholes or manholes (if installation is done in an existing duct trajectory, where digging the street again is to be avoided).
During blowing/pushing installation of a bundle of guide tubes, the propelling air-drag force developed by the volumetric flow of air through the duct is proportional to the compressor output pressure and bundle diameter. However, the frictional load imposed by rubbing engagement is proportional to the bundle weight, hence to the square of the bundle diameter. Moreover, a bundle that fills the duct for a large part, when the bundle is tight, is subjected to extra friction caused by bends and undulations in the duct due to the stiffness of the bundle, which increases with the fourth power of the bundle diameter. On the other hand a bundle that just fits in the protective duct can be pushed harder without buckling, but the frictional loading caused by rubbing engagement of the guide tubes against the protective duct imposes a limit on the continuous installation length that can be obtained by pushing/blowing for such large bundle diameters.
It has been demonstrated during field trials that when the filling degree of the guide tube bundle (sum of guide tube cross-sectional areas compared to that of protective duct) is provided in the range of from about 30% to about 60%, blowing/pushing installation of the guide tube bundle is substantially trouble free. Also, unexpectedly, substantially longer continuous runs were accomplished in less time than required for relatively large diameter guide tube bundles. The selection of the diameter and number of guide tubes to provide a partial filling factor of not more than about 50%, with the guide tubes arranged in a loose bundle, provided the best results. The partial filling of guide tubes arranged in a loose bundle also provides good mechanical protection. By limiting the bundle diameter so that the guide tubes only partially fill the duct air flow space, preferably by a filling factor of about 50% (half of duct cross-sectional area) and not more than about 60%, the guide tubes and hence any cables contained therein are protected just as good as armored cables, because the loose guide tubes can move away when the duct is indented. Also, by installing the guide tubes as a loose bundle as opposed to a solid bundle, the guide tubes are more easily accessible at branch points which are formed as new subscribers are added. Further, a loose bundle of guide tubes has a reduced stiffness factor as compared with the same guide tubes arranged in a tightly bound bundle.
The invention is based on the insight that the propelling drag force needed to move a loose bundle of guide tubes (with or without cables) through a duct by the volumetric flow of compressed air is generally proportional to the effective external surface area presented by the guide tubes, which is in turn proportional to the effective diameter of the guide tube bundle. At the same time, the volumetric flow rate of compressed air through the protective duct is limited by the output pressure of the air compressor, which is generally constant, by the diameter of the duct (which is fixed) and by the flow resistance imposed by the mass of the guide tube bundle. The guide tube bundle throttles air flow through the surrounding bundle/duct annulus as the bundle diameter increases relative to the duct diameter.